This invention relates to the fabrication of piezoelectric crystal devices, and in particular to a method of tuning the crystal to produce highly stable frequencies and a resulting product.
Piezoelectric crystal devices, especially those comprising quartz, are presently used in a variety of applications such as filters and oscillators. A significant step in the fabrication of such devices is tuning of the crystal to achieve a precise resonant frequency. The crystal is typically fabricated to a dimension which produces a resonant frequency less than the desired frequency and the frequency is then adjusted upward by reducing the frequency determining dimensions of the crystal. The techniques most widely used for removing quartz material involve abrasive removal such as sandblasting (abrasive jet machining) or grinding with abrasive paper. Although these techniques are adequate for most applications, they have two primary disadvantages. First, abrading causes a certain amount of surface damage to the crystal in the form of a dense arrangement of microcracks. Such damage causes strains in the crystal which usually result in frequency instability and higher motional resistance. Further, abrasive removal usually leaves some contamination on the surface of the crystal which in turn requires additional cleaning steps after the initial tuning. These effects often require repeated frequency tuning, and a frequency within tight tolerances is difficult to obtain. In addition, the frequency of the crystal may change after it has been mounted in a carrier. At this point, abrasive removal becomes impractical and the devices are typically tuned by adding mass to the electrodes on the plate. This process can be expensive or impractical however, since metal deposition is usually performed by evaporation in a vacuum, or by electroplating with a plating solution which can cause frequency instability therefore requiring subsequent stabilzation steps.
It is therefore a primary object of the invention to provide a means for adjusting the resonant frequency of crystal plates so that the frequency is highly stable. It is a further object of the invention to provide a means which permits adjustment of frequency within tight tolerances. A yet further object of the invention is to provide a convenient means for adjusting the frequency of the crystal after its incorporation into a device structure.